Chicken eggs are known to be excellent foods that have well-balanced amino acid compositions and contain lipids. In addition, chicken eggs and fish eggs are both known to be foods with high cholesterol content. It is said that the cholesterol content of a chicken egg is 1,300 to 1,400 mg per 100 g of egg yolk on average and 450 to 500 mg per 100 g of a whole egg (an edible part) on average. According to the average values above, a chicken egg contains about 230 mg of cholesterol (Non-Patent Document 1).
It is said that most cholesterol (80% or more) in the human body is not derived from food but rather is biosynthesized in the human body. As dietary cholesterol intake increases, in general, the mechanism for suppressing cholesterol biosynthesis in the body is activated, and cholesterol homeostasis in the blood is maintained. Thus, according to the mainstream way of thinking, a healthy individual does not need to pay much attention to the dietary cholesterol intake from chicken eggs (Non-Patent Document 2).
It is known that some persons easily reflect dietary cholesterol intake in their blood cholesterol levels (those who are referred to as “hyper-responders”) while others do not (those who are referred to as “hypo-responders”) (Non-Patent Documents 3 and 4). During the treatment of hyperlipidemia, the upper limit of the amount of cholesterol that a patient is allowed to ingest in a day may be determined to be approximately 300 mg (Non-Patent Document 5). Accordingly, it is important to know the cholesterol content of food materials, such as chicken eggs, for the purpose of keeping the amount of cholesterol ingested within an adequate range.
As a method for regulating the cholesterol level of a chicken egg, for example, a method of feeding dry natto to a chicken is known (Non-Patent Document 6). In the case of usual chicken eggs, however, cholesterol levels are known to vary significantly among individuals, such as from about 1,000 mg to more than 1,800 mg per 100 g of egg yolk. Therefore, a means that enables measurement of the cholesterol content of a chicken egg in a simple manner has been desired.
For example, Non-Patent Document 7 discloses, as a method for measuring the cholesterol content of a chicken egg, a method comprising deproteinization of egg yolk or whole egg, extraction of a cholesterol-containing fraction, and analysis of the cholesterol content in the extract via high-performance liquid chromatography (hereafter, it may be referred to as “HPLC”). Also, Non-Patent Document 8 discloses an enzymatic method of measuring the cholesterol content in the cholesterol-containing extract with the use of cholesterol oxidase or catalase.
When a measurement sample is prepared by breaking the target chicken egg as described in the aforementioned documents, the target chicken egg cannot be used as a food material. In order to measure the cholesterol content of a chicken egg to be used as a food material, accordingly, it is necessary that the measurement be carried out by a non-destructive means. As an example of a method for characterizing features of a chicken egg by a non-destructive means, Patent Document 1 discloses a method of processing eggs having identified characteristics comprising steps of: irradiating an egg with light from a light source, wherein the light includes light in both the visible and infrared wavelength ranges; receiving light transmitted through each egg with a detector positioned adjacent to each egg; generating a spectrum for each egg that represents light intensity at a plurality of the visible and infrared wavelength regions; and comparing the generated spectrum for each egg with a spectrum associated with a live egg to identify live eggs.